Designation B70 − 90 (Reapproved 2013) Standard Test Method for Change of Resistance With Temperature of Metallic Materials for Electrical Heating1 This standard is issued under the fixed designation[.]
Trang 1Designation: B70−90 (Reapproved 2013)
Standard Test Method for
Change of Resistance With Temperature of Metallic
Materials for Electrical Heating1
This standard is issued under the fixed designation B70; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision A number in parentheses indicates the year of last reapproval A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1 Scope
1.1 This test method covers the determination of the change
of resistance with temperature of metallic materials for
elec-trical heating, and is applicable over the range of service
temperatures
1.2 The values stated in SI units are to be regarded as
standard No other units of measurement are included in this
standard
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use It is the
responsibility of the user of this standard to become familiar
with all hazards including those identified in the appropriate
Material Safety Data Sheet (MSDS) for this product/material
as provided by the manufacturer, to establish appropriate
safety and health practices, and determine the applicability of
regulatory limitations prior to use.
2 Significance and Use
2.1 The change in resistance with temperature for heating
element materials is a major design factor and may influence
material selection The measurement of this change is essential
to ensure that heating elements perform as designed This test
method was designed to minimize the effect different
manu-facturing processes have on resistance change, thereby yielding
results that are reproducible
3 Test Specimen and Leads
3.1 The test specimen shall be prepared from material as left
by the manufacturing process, and in a form suitable for
measuring its resistance in an electric furnace When the
resistance is to be measured with a Kelvin bridge,
potentiometer, digital ohmmeter, or equivalent, a current lead
shall be welded to each end of the specimen in such a manner
that there will be no change of current distribution in the
specimen during measurements Potential leads, one at each
end, shall be attached by welding, at a distance from the corresponding current lead not less than one tenth of the length
of the specimen between the potential leads
3.2 When the resistance is to be measured with a Wheat-stone bridge, only the current leads are required The resistance
of the leads in this case shall not exceed 1 % of the resistance
of the specimen and the leads shall be made of the same type
of alloy as the test specimen For both methods of measurement, the leads shall have a length within the heated zone of the furnace of at least 50 times their minimum transverse dimension, in order to avoid disturbance of the temperature of the specimen by conduction of heat to the colder parts of the furnace
4 Electric Furnace
4.1 The furnace for heating the specimen shall be of such a type that the temperature can be controlled over the range from room temperature to the maximum desired It shall be so constructed that the specimens and the thermocouples can be maintained at a uniform and constant temperature at desired points within the working range The specimen and thermo-couples shall be so shielded as to prevent direct radiation from hotter, or to colder, parts of the furnace
4.2 In order to test the uniformity of the temperature in the region to be occupied by the test specimen, a typical specimen and thermocouple shall be prepared and mounted in the center
of this region The furnace shall then be heated to its maximum temperature and maintained at this temperature until equilib-rium is reached The specimen shall then be moved in the furnace in the direction of the maximum temperature gradient through a distance equal to the maximum dimension of the largest specimen and thermocouple assembly which is to be used in this furnace The temperature of the typical specimen in this position shall not differ from that in the normal position by more than 10°C
5 Resistance Measurements
5.1 A Kelvin bridge, potentiometer, digital ohmmeter, or equivalent shall be used when measuring specimens having resistances less than 10 Ω A Wheatstone bridge may be used with specimens having resistances greater than 10 Ω The resistance of the specimen shall be measured with an accuracy
1 This test method is under the jurisdiction of ASTM Committee B02 on
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
B02.10 on Thermostat Metals and Electrical Resistance Heating Materials.
Current edition approved May 1, 2013 Published May 2013 Originally
approved in 1927 Last previous edition approved in 2007 as B70 – 90 (2007) DOI:
10.1520/B0070-90R13.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 United States
1
Trang 2of 0.1 % The measuring current shall be so small that the
resistance of a specimen is not changed thereby as much as
0.1 % This condition may be determined experimentally or
calculated from the power expended and the surface of the
specimen
6 Test Current
6.1 To determine experimentally that the test current is not
too large, bring the specimen to a temperature (Note 1) where
there is a relatively large uniform change of resistance with
temperature Apply the test current and maintain it until the
resistance of the specimen has become constant Then increase
the current by 40 % and maintain it at this value until the
resistance has again become constant If the change in
resis-tance is greater than 0.1 %, the test current is too large and shall
be reduced until the foregoing limitations are reached
N OTE 1—In the case of nickel-chromium alloy 400°C is a suitable
temperature.
6.2 The test current has a negligible effect on the resistance
measurement when the power lost in its passage through the
specimen is less than 0.01 W/cm2of the effective free surface
of the specimen For straight specimens and those which are so
coiled or bent that the distance between the adjacent
convolu-tions is greater than five times the maximum transverse
dimension of the cross section of the specimen, consider the
free surface to be that surface area of the portion of the
specimen between the potential leads When, as for the sake of
saving space in the furnace, the specimen is wound into a spiral
or helix, or bent back and forth upon itself in such a manner
that the distance between adjacent convolutions is less than five
times the maximum transverse dimension of the cross section
of the specimen, consider the free surface to be that surface
area of the cylindrical or prismatic volume enclosing the coiled
or convoluted specimen Calculate the power loss due to the
measuring current from the following equation:
W 5 I 2 R m
where:
W = power loss, W,
I = measuring current A, and
R m = resistance at maximum test temperature, Ω
7 Procedure
7.1 Mount the test specimen in the furnace, bring the
temperature of the furnace to the maximum specified
tempera-ture of test for the alloy in question, and hold at this value until the resistance of the specimen remains constant except for the relatively slow changes due to oxidation Then lower the temperature of the furnace in steps of approximately 100°C to room temperature Take measurements at each point when temperature and resistance have become stationary Note the time at which each reading is taken Define each point by the mean of at least one pair of resistance readings for which the current through the sample has been reversed between read-ings This is necessary in order to eliminate the effects of thermal electromotive forces
7.2 Determine the temperature by means of calibrated thermocouples, in conjunction with a potentiometer or pyrom-eter of such construction as to ensure an accuracy correspond-ing to a temperature uncertainty not exceedcorrespond-ing 10°C
8 Temperature-Resistance Curve
8.1 Plot a curve showing the change of resistance with temperature using the final room-temperature resistance value
as a base Note the time interval between successive readings
on the curve sheet Consider the curve thus obtained with descending temperature as defining the true temperature-resistance characteristics of the material tested
9 Singular Points
9.1 If there are indications that the curve is not smooth at any point, carry the specimen through the temperature cycle again, and take temperature and resistance readings at intervals
of approximately 25°C in the suspected region
10 Precision and Bias
10.1 The reproducibility of the change in resistance with temperature depends primarily on the uniformity of tempera-ture of the sample and secondarily on the rate of temperatempera-ture decrease For nickel-chromium alloys the faster the decrease in temperature, the lower the change in resistance
10.2 The precision of this test method is within 62 % 10.3 The bias cannot be determined for reasons detailed in
10.1
11 Keywords
11.1 coefficient of resistance; heating elements; resistance change; resistors; temperature-resistance
ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned
in this standard Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk
of infringement of such rights, are entirely their own responsibility.
This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and
if not revised, either reapproved or withdrawn Your comments are invited either for revision of this standard or for additional standards
and should be addressed to ASTM International Headquarters Your comments will receive careful consideration at a meeting of the
responsible technical committee, which you may attend If you feel that your comments have not received a fair hearing you should
make your views known to the ASTM Committee on Standards, at the address shown below.
This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,
United States Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the above
address or at 610-832-9585 (phone), 610-832-9555 (fax), or service@astm.org (e-mail); or through the ASTM website
(www.astm.org) Permission rights to photocopy the standard may also be secured from the ASTM website (www.astm.org/
COPYRIGHT/).
B70 − 90 (2013)
2